My catamaran has transom-hung outboards. I'm thinking about the best location for the starting batteries and permanently installed below deck fuel tanks. We currently use portable tanks above deck. They are always tripping people, fouling sheets, or otherwise in the way. Each hull has a compartment at the stern with a bulkhead sealing it off from the rest of the boat. These compartments would be the perfect size and location for battery and tank. Assuming the battery is installed correctly (strapped down in leakproof box; vented; no ignition sources, etc) and assuming a permanent below deck tank is installed correctly (strapped down; properly installed deck fill and vent, etc), would it be illegal/unsafe/unwise to have battery and gasoline tank side-by-side in the same compartment?

Fires and explosions require three conditions before they can occur. This is called the fire triangle. It is, oxygen, a source of ignition, and fuel. On boats the oxygen obviously is present, the source of ignition is usually the boats electrical system, and the fuel is generally gasoline. So, if one side of the fire triangle can be eliminated, then the explosion and fire can be prevented. Each of the standards dealing with fuel systems, electrical systems and ventilation is designed to eliminate one of the sides of the fire triangle.

Kind of simple. But, it makes sense. Would it be possible to build a glassed-in box that is ventilated to the outside for the gas tank?

Well, I've read all the AYBC/USCG/ISO regs I can find regarding battery installation and permanent fuel tank installation. AYBC says the battery can't be directly over or under the fuel tank, and there are specific rules about fuel hoses running over batteries. Otherwise I can't find anything specifically about battery/fuel tank proximity. The only issue I know of is that all electrical devices in the compartment have to have ignition protection certification. I guess batteries don't have that certification, even if under normal conditions they are not an ignition source. So the batteries will have to move to another home.

Battery s are allowed but as i said before i would consult with a good survey person before making such a big change as you have no motor down there the ventilation requirements are the issue that's up in the air

How big are these compartments? are they just lockers or are they someplace you could install inboards if desired?
My batteries are about 3ft from the fuel tank, diesel now was gas, in the engine bay, all that seperates them is a peg board and 3ft.
can you split the compartment in some way to devide it as a removeable bulkhead?

With the safety precautions already noted above by others, here's another reason your idea may not be so good. I'm not certain how large your catamaran is but fore-aft hull trim is key to catamaran performance, and so you may not want to concentrate too much weight in aft. compartments. Both fuel and batteries are heavy and might cause your transoms to drag.

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